The method for the formation of multi-layer paint films on an object which is to be painted with a three-coat one-bake system comprising the processes 1 to 5 indicated below using an aqueous first colored paint (A), an aqueous second colored paint (B) and a clear paint (C) is known as a method for the formation of aqueous three-coat one-bake (3C1B) multi-layer films which have a good finish without the formation of runs, unevenness or interlayer mixing. Process 1: A process in which an aqueous first colored paint (A) comprising urethane emulsion (a) and other components (b) is applied. Process 2: A process in which the paint film viscosity of the aqueous first colored paint (A) is set to at least 1×103 Pa·sec (20° C., shear rate 0.1 sec.−1) by setting at normal temperature. Process 3: A process in which an aqueous second colored paint (B) is applied. Process 4: A process in which preliminary heating is carried out. Process 5: A process in which a clear paint (C) is applied and then the paint film comprising three layers is all baked and dried at the same time. (For example, see Patent Citation 1) However, with this method little consideration is given to the paint film properties and there is a weakness in that the chipping resistance is poor.
Furthermore, a method for forming a multi-layer paint film in which an aqueous mid-coat paint (1), an aqueous base paint (2) and a clear paint are coated sequentially wet-on-wet on a base material on which an electrodeposited paint film has been formed and the multi-layer paint film obtained is all baked and hardened at the same time, which is characterized in that the aforementioned aqueous mid-coat paint (1) includes an acrylic emulsion of average particle diameter from 0.05 to 10 μm, a urethane emulsion of average particle diameter from 0.01 to 1 μm and a curing agent, the average particle diameter of the acrylic emulsion is the same as, or greater than, the average particle diameter of the urethane emulsion and the acrylic emulsion/urethane emulsion number ratio is from 1/0.1 to 1/500, is known as a method in which the impact resistance and appearance of the paint film are improved using an aqueous paint for both the mid-coat and base paint in a 3C1B painting system (for example see Patent Citation 2). However, there is a weakness with this method in that the improvement in the chipping resistance is inadequate.
Furthermore, a method for the formation of multi-layer paint films which includes (1) a process in which an object to be painted on which an electrodeposited paint film has been formed is provided; (2) a process in which an aqueous mid-coat paint is applied over the electrodeposited paint film and a mid-coat paint film is formed; (3) a process in which an aqueous base paint, and a clear paint, are coated sequentially wet-on-wet over the mid-coat paint film without curing the mid-coat paint film and a base paint film and a clear paint film are formed; and (4) a process in which the mid-coat paint film, the base paint film and the clear paint film are all baked and cured at the same time, which is characterized in that said aqueous mid-coat paint contains acrylic resin emulsion, curing agent and titanium oxide pigment which has been treated with organic material, is known as an aqueous 3C1B multi-layer paint film forming method with which the properties of the mid-coat paint film are improved and the chipping resistance of the multi-layer paint film is improved (for example, see Patent Citation 3). However, with this method the mid-coat paint film of the first layer is of a light color since it contains much titanium oxide which is a white pigment, and in those cases where the multi-layer paint film has a deep color it must be colored with just the second layer and the reproducibility of the paint color is unstable, and there is a further weakness in that the chipping resistance is also inadequate.
Furthermore, a method for the formation of a multi-layer paint film in which a colored first base paint (A) and a colored second base paint (B) are applied and then a clear paint (C) is applied wet-on-wet, after which the multi-layer paint film comprising the three layers is all heated and crosslink-cured at the same time, which is characterized in that a colored first base paint (A) of which the Young's modulus of the cured paint film of the colored first base paint (A) at −20° C. is at least 3000 MPa and the breaking energy is at least 2×10−3 J, is known as a 3C1B multi-layer paint film forming method with which the paint film property values include good chipping resistance where a paint which has good chipping resistance has been developed (for example, see Patent Citation 4). However, with this method the paint film properties of the first layer are specified but the means by which this is achieved are unclear, and according to the detailed description of the invention it is impossible to reach the specified paint film values unless a solvent-based paint which gives rise to problems with the VOC discharge regulations is used for the first layer and there is a problem in that good chipping resistance is not obtained.